To investigate the correlation and agreement between the intraocular pressure peaks detected during the water drinking test and modified diurnal tension curve in glaucomatous eyes and to analyse whether this is associated with prognostic outcome. The study will also measure autonomic nervous system activation during the test to investigate one possible mechanism behind it.

Glaucoma is a progressive optic neuropathy characterized by progressive loss of retinal ganglion cells, leading to thinning of the retinal nerve fiber layer and visual field losses. It is the leading cause of irreversible blindness in the world [2] and affects more than 70 million people worldwide. Intraocular pressure (IOP) is the main risk factor for the development and progression of glaucoma. The gold standard for intraocular pressure (IOP) measurement is applanation tonometry. Routine office measurements may not detect IOP peaks in roughly 30% of patients due to variation throughout the day and this detection failure may be responsible for visual field progression in apparently well-controlled patients. Therefore this study is to investigate the correlation and agreement between the intraocular pressure peaks detected during the water drinking test and modified diurnal tension curve in glaucomatous eyes and to analyze whether this is associated with prognostic outcome. The study will also measure autonomic nervous system activation during the test to investigate one possible mechanism behind it. The primary measure is peak Intraocular pressure measurements from two different measures. Secondary measures include OCT angiography, postural blood pressure, heart rate, pupil diameter. Current and future rates of progression using visual field and optical coherence tomography (OCT) measures. This is a prospective method comparison study, of 40 primary open angle and normal tension glaucoma patients who will attend for a single visit. Inclusion criteria include those

• Able to give informed consent to participate in the study
• Aged between 18 and 80 years of age
• Clear optical media
• Spherical equivalent +-10 Dioptres
• Normal tension glaucoma proven by visual field testing or OCT imaging combined with clinical assessment.
• Primary open angle glaucoma proven by visual field testing or OCT imaging combined with clinical assessment

Following completion of the study, participants will be followed up in their usual clinic as part of their regular care. The participants' follow-up data which is routinely generated as part of the glaucoma care pathway will be accessed for 18 months following completion of participation with a view of correlating initial results with possible disease progression and other related outcomes.

Inclusion Criteria:

• Able to give informed consent to participate in the study
• Aged between 18 and 80 years of age
• Clear optical media
• Spherical equivalent +-10 Dioptres
• Normal tension glaucoma proven by visual field testing or OCT imaging combined with clinical assessment.
• Primary open angle glaucoma proven by visual field testing or OCT imaging combined with clinical assessment.

Exclusion Criteria:

• Are using ocular hypotensive therapy, or have done so in the last 30 days
• Have any other known ocular disease (except glaucoma)
• Have serious cardiac or kidney disease (WDT contra-indication)
• Have swallowing difficulties associated with either a neurological or gastrointestinal condition.
• Patients with any contra-indication of drinking large amounts of water in a small period of time e.g. swallowing impairment, oesophageal stricture, gastric banding.
• Have secondary or narrow/closed angle glaucoma
• Patients submitted to any surgical procedure or laser intervention during the evaluation period

Primary open-angle and normal tension glaucoma patients

2.5 months